1. Field of the Invention
The present invention relates to a chemical and mechanical polishing (hereinafter simply referred to as CMP) apparatus for flattening or planarizing a surface of an object such as a wafer.
2. Description of the Related Art
In recent years, as manufacturing of semiconductor devices with high density and high integration advances, ultra-large-scale integrated (hereinafter simply referred to as ULSI) circuit devices having great capacities becomes popular. In the ULSI manufacturing process, a technique of flattening a multi-layer film is required, and the CMP apparatus is used for that purpose.
FIG. 11 is a sectional view showing one example of a surface flattening operation with such a CMP apparatus.
When a surface of an object in the form of a wafer 100 comprising a silicon substrate 101 and an oxide film 102 of SiO.sub.2 formed thereon is polished by using a CMP apparatus as shown in FIG. 11(a), rugged or irregular portions of the oxide film 102 are removed so as to flatten or planarize the surface thereof as shown in FIG. 11(b).
Since the wafer 100 actually involves warpage and/or irregularities or variations in its thickness as shown in FIG. 12, however, it is quite difficult to flatten the wafer 100 by use of the CMP apparatus to any satisfactory extent.
Thus, the flattening of the wafer 100 having warpage, etc., was done by the conventional CMP apparatus using a system shown in FIGS. 13(a) and 13(b).
FIG. 13(a) is a sectional view showing surface flattening due to a so-called rear surface reference method. In FIG. 13(a), reference symbol 1 designates a carrier which has a holder 2 for holding an object to be polished in the form of a wafer 100 and a carrier shaft 3 fixedly coupled to a cone-shaped outer surface of the holder 2, the carrier 1 serving to firmly hold the wafer 100 on a surface plate 300 disposed under the carrier 1 during a polishing operation.
More specifically, a hard pad 200 made of foam urethane is attached or adhered to an inner surface of the holder 2 disposed on the opposite side of the carrier shaft 3. The hard pad 200 has a lot of holes 201 in communication with corresponding branches of an air passage 40 formed through the body of the holder 2 and the carrier shaft 3.
Moreover, a soft pad 210 and a hard polishing pad 220 are secured or adhered to an upper surface of the surface plate 300 in a laminated or stacked manner. The soft pad 210 may be in the form of a non-woven fabric pad such as one commercially available under the trade name of "Suba 400", and the hard polishing pad 220 may be in the form of a polishing pad made of foam urethane such as one commercially available under the trade name of "IC 1,000".
With this arrangement, a lower surface of the hard pad 200 attached to the carrier 1 is made in contact with the wafer 100 so that air in a space between the lower surface of the hard pad 200 and the wafer 100 is evacuated through the air passage 40 in the carrier 1 by means of an unillustrated air pump connected to the air passage 40 whereby the wafer 100 is sucked or attracted under vacuum to the lower surface of the hard pad 200. Then, the wafer 100 thus fixedly held by the carrier 1 is transferred to a position over the hard polishing pad 220 of the surface plate 300, where the wafer 100 is made in contact at its lower surface with the hard polishing pad 220 through a downward motion of the carrier 1. Thereafter, the suction force applied to the wafer 100 is released, allowing the wafer 100 on the hard polishing pad 220 to move relative to the carrier 1 and hence the hard pad 200 with the result that the opposite surfaces of the wafer 100, which is being urged against the hard polishing pad 220 by means of a downward force or movement of the carrier 1, are polished by means of the hard polishing pad 220 of the rotating surface plate 300 and the hard pad 200 of the carrier 1 which is rotating in a direction opposite a rotational direction in which the surface plate 300 is rotating.
In this case, the rear or second surface 100b of the wafer 100 is pressed so as to be flat by means of the hard pad 200, so that influences of warpage and/or irregularities in the thickness of the wafer 100 appear at the front or first surface 100a of the wafer 100.
However, since there exists the soft pad 210 on the side of the front or first surface 100a of the wafer 100, the hard polishing pad 220 is deflected together with the soft pad 210 along the curved contour of the front surface 100a. As a result, the surface 100a of the wafer 100 is polished by a constant thickness and flattened by means of the hard polishing pad 220.
On the other hand, FIG. 13(b) is a sectional view showing surface flattening due to a so-called front surface reference method. In this method, to the inner surface of a holder 2 there is attached or adhered a soft pad 230 in the form of a suede-type pad such as, for example, one commercially available under the trade name of "R200" which has a lot of through-holes 231 with which an air passage 40 is in communication through a plurality of corresponding branches in the holder 2. A hard polishing pad 220 is secured or adhered to the surface plate 300.
With this arrangement, the front surface 100a of the wafer 100 is urged or pushed so as to be flat by the hard polishing pad 220, and influences of warpage and/or irregularities in the thickness of the wafer 100 appear on its rear surface 100b. However, since there exists the soft pad 23 on the rear surface 100b of the wafer 100, the soft pad 230 is deformed along the curved contour of the rear surface 100b of the wafer 100. Consequently, the flat surface 100a of the wafer 100 is polished by a constant amount of thickness and thus flattened by means of the hard polishing pad 220.
However, the above-mentioned CMP apparatuses involve the following problems.
The first problem is that the elastic recovery rates of the hard pad 200 and the soft pad 230 attached to the holder 2 are low or poor. That is, the elastic recovery rate of the pad made of foam urethane used in general as the hard pad 200 is about 80%, so the foam urethane pad could not return to the original shape after a short period of use, and hence the duration or life thereof is short. Especially, the elastic recovery rate of "R200" used in general as the soft pad 230 varies within the range from 70% to 98% depending upon goods manufactured. Therefore, the soft pad 230 in the form of a product with a bad or low elastic recovery rate is not only of a short life, but also the polishing accuracy thereof will be deteriorated in a short period of use in other words, the soft pad 230 elastically deforms along the curved contour of the rear surface 100b of the wafer 100 so as to allow the surface 100a of the wafer 100 to be subjected to a uniform pressure, whereby the wafer surface 100a can be polished in a uniform manner while being held in a substantially flat state.
However, the soft pad 230, when the elasticity thereof has been deteriorated, can not deform along the curved contour of the rear surface 100b of the wafer 100, as a consequence of which the pressure or urging force applied from the soft pad 230 to the wafer 100 will become irregular or nonuniform, making the polishing speed of the wafer 100 different at portions thereof.
The second problem is that a lot of dents or recesses in the shape of dimples are created on the surface 100a of the wafer 100. That is, through-holes 201, 231 of diameters in a range from 1 mm to 2 mm which communicate with the corresponding branches of the air passage 40 are formed through the hard pad 200 and the soft pad 230, as shown in FIG. 13(a) and FIG. 13(b). When the carrier 1 transports a wafer 100 onto the surface plate 300, air in a space between the carrier 1 and the wafer 100 is discharged or evacuated via the through-holes 201 or 231 in the pad 200 or 230 and the air passage 40 by means of the unillustrated air pump connected with the air passage 40, so that the wafer 100 is sucked or attracted to the hard pad 200 or the soft pad 230 of the carrier 1 under the action of a vacuum. When the wafer 100 is polished, the suction or attractive force acting on the wafer 100 is liberated so that the wafer 100 is not partially sucked into the through-holes 201 or 231. However, a great pressure of 500 g/cm.sup.2 is applied downward to the wafer 100 during polishing operation. As a result, those portions of the wafer 100 which are located at through-holes 201 or 231, as shown in FIG. 14, are pushed into the through-holes 201 or 231, thus creating recesses or dents 100c in the shape of dimples on the surface 100a of the wafer 100.
The third problem is that the wafer 100 might be dashed or flied out from the carrier 1 and damaged while the wafer 100 is polishing. That is, since the carrier 1 rotates at high speed while holding the wafer 100 with the surface plate 300 being also rotating, there might be the case that the wafer 100 occasionally happens to dash or fly out from the carrier 1 for some reason. In order to cope with this, it is preferred that air in the space between the wafer 100 and the pad 200 or 230 of the carrier 1 be evacuated or discharged through the through-holes 201 or 231 and the air passage 40 in the carrier 1 to thereby draw the wafer 100 against the carrier 1 even during polishing of the wafer 100. If, however, the drawing or suction force acts on the wafer 100 via through-holes 201 or 231, the dimple-shaped dents 100c in the wafer 100 grow deeper or greater, further worsening the surface state of the wafer 100.
The fourth problem is that it is difficult to afford a desired magnitude of elasticity to the wafer 100. For instance, as shown in FIG. 13(a), the apparatus is constructed such that it includes one hard pad 200 made of foam urethane, one soft pad 210 in the form of "Suba400", and one hard polishing pad 220 in the form of "IC1,000". However, there sometimes arises a case that the thickness of each pad is desired to be varied so as to change the amount of deformation thereof depending upon the kind or type of the wafer 100 to be polished. Since the thickness of each of these pads is standardized and can not be varied, however, two or more hard pads 200 are used which are secured to the inner surface of the holder 2 in a laminated or stacked manner, and two or more soft pads 210 are secured to the surface of the surface plate 300 beneath the single hard polishing pad 220 similarly in a laminated or stacked manner. In this manner, lamination of a plurality of pads each having a fixed or same thickness results in a discrete value of the total thickness of the pads, so it is difficult to set the total thickness of the laminated pads to a desired value, thus making it difficult to adjust a slight amount of deformation of the pads.
The fifth problem is that the composition of the pad on the rear surface of the wafer 100 is changed by a polishing agent such as slurry. That is, chemical substances such as alkaline materials and acidic materials might be included in the slurry which is used as a polishing material, and the composition of the hard pad 200 or the soft pad 230 can be changed by such chemical substances, resulting in deformations and/or changes in the hardness and elasticity of the pad.